1. Field of the Invention
This invention relates to the field of integrated circuit devices and in particular to integrated circuit having grid devices.
2. Background Art
Miniature, micrometer-sized vacuum tube devices, fabricated on a semiconductor wafer using integrated circuit fabrication techniques, are known. For example, miniature vacuum diodes and vacuum triodes are described in a paper entitled "Modeling and Fabricating Micro-Cavity Integrated Vacuum Tubes", William J. Orvis, et al., IEEE Transactions on Electron Devices, Volume 36, No. 11, November 1989, pages 2651-2658. An array of microelectronic tubes is disclosed in U.S. Pat. No. 4,721,885, issued Jan. 26, 1988 to Brodie. A split collector vacuum field effect transistor is disclosed in U.S. Pat. No. 5,012,153, issued Apr. 30, 1991 to Atkinson, et al.
These devices utilize a single point field emitter in conjunction with at least one grid having an opening which is spaced from and concentric with the point of the field emitter. In such devices, ion-bombardment damage or sputtering of the field emission tip can be a serious problem. Additionally, the usefulness of such devices is limited because of the low level of electron flow which is possible from the field emission tip. Furthermore, these devices, do to their configuration, require relatively large amounts of chip area.
Thus it is desirable to provide a way to obtain more reliable integrated circuit grid devices having inversed electron flow. One solution to this problem is the use of hot-electron thin film devices which are known in the art. For example, the "Handbook of Thin Film Technology", edited by Leon I. Maissel and Reinhard Glang, McGraw-Hill, Inc., 1970, discloses a tunnel-cathode emitter. The tunnel cathode is based on the fact that the electron energy is conserved during the tunnel process. Thus in tunneling from one electrode to the other, such that eV&lt;d.sub.o, the electrons enter the positively biased electrode with an energy level eV above the Fermi level of the electrode. The electrons then give up their energy to the lattice and fall into the Fermi sea.
For voltage biases such that eV&gt;d.sub.o, the electrons first tunnel into the conduction band of the insulator before entering the positively biased electrode. The electron is then assumed to be accelerated by the field within the insulator, without undergoing energy losses, to again enter the positively biased electrode with energy eV above the Fermi level. If eV is less than the positively biased electrode work function C, the electron gives up its energy to the electrode lattice as previously described. However, if eV&gt;C and less than the mean free path of the electrons, the electrons may pass through the electrode to the vacuum interface with little loss of energy, and thus escape into the vacuum.
It is thus believed that with a suitable geometry and voltage bias, a large fraction of the tunneling electrons should be able to escape from the tunnel junction into the vacuum and be collected by a suitably biased anode. The tunnel junction then, in principle, is a cold cathode. However, such cathodes are extremely inefficient, having transfer ratios typically on the order of 10.sup.-4 or 10.sup.-3 wherein the transfer ratio is understood to be the ratio of emission current to circulating current. These low values of the ratio of emission apparently result from the fact that the majority of electrons undergo energy losses while traveling in the conduction band of the insulator and the metal film. The attenuation of electrons appears to be directly related to the square of the thickness of the metal film.
It is also known to provide a tunnel-emission triode wherein a second insulator, assumed to be less than the electronic mean free path, and a third electrode are deposited onto the cold cathode. In the tunnel emission triode, the energy of electrons tunneling between the emitter and base electrode is assumed to be conserved when they reach the interface existing between the base and the second insulator. At this point electrons may not have sufficient energy to enter the conduction band of the second insulator. If the energy level is high enough electrons can enter the conduction band of the second insulator, in which case they are then accelerated toward and collected by the collector which is positively biased with respect to the base during operation. Thus the second insulator and collector serve the same function as the vacuum interspace and anode in the cold-cathode emitter.
However, the tunnel emission triode suffers from all the disadvantages of the cold cathode, plus additional problems arising from scattering and trapping in the collector insulator, which are not present in the vacuum interspace between the tunnel junction and anode comprising the cold cathode.
It is well known in the art that at high temperatures the properties of the semiconductor materials which form the semiconductor integrated circuits change, causing devices to operate improperly. It is known in the art to cool such devices in order to maintain their performance under conditions in which their temperature would be raised above the operating limit. This permits these devices to be operated with more watts per square centimeter than a similar circuit without cooling. However, the cooling of these integrated circuit devices can be a serious drain on resources and a serious limitation on what can be accomplished using these chips.
It is also known in the art that semiconductor devices are sensitive to transient radiation because of bulk generation of charge carriers in the active regions of these devices. For example, alpha particles can cause a charge which can latch up a device. These charge carriers tend to negate the topology of transitions and render them inoperable for the duration of the transient.
It is also known in the art to use a silicon micro-machining procedure to fabricate micro-cavity integrated vacuum tubes. This procedure can be performed with known integrated circuit processing equipment. The cathode, grid and anode of the vacuum tube is fabricated using planner technology so that the interconnection of many devices can be easily achieved. Low temperature chemical deposited oxide is used to separate the grid from the cathode and to separate the anode from the grid. Low temperature chemical vapor deposited oxide is also used as a sacrificial layer to etch cavities in the area of the field emitting points. Grid openings of a micron and registration accuracy between layers of 0.1 micrometers have been achieved. These devices use field emission rather than thermionic emission to generate charged carriers. All this is useful because miniature vacuum tubes are more radiation and temperature tolerant. This is useful in fission reactors, fusion reactors, and accelerators having instrumentation, control, and power conditioning electronics which are subjected to high temperatures and radiation fields.
These devices consist of a silicon field-emission pyramid on a silicon substrate. The pyramid is created by anisotropic etching of silicon. The field emitter is buried in the layer of phosphorous-doped silicon dioxide glass which is reflowed to make it more planer. Above the glass a pattern strip of doped polysilicon is deposited. The strip has a hole in it centered over the field emitter. In this device ion-bombardment damage or sputtering of the field emission tip is a serious problem. Additionally, the usefulness of these devices is limited because of the low level of electron flow which is possible from the field emission tip. Furthermore, voltages on the order of 50 volts to 150 volts are required to operate these devices.